1. Field of the Invention
The present invention relates to a cushioning pad used for hot pressing, a rubber for the cushioning pad and manufacturing method thereof. Further, the present invention relates to a method of manufacturing a printed circuit board, an IC card, a liquid crystal display panel and the like and more preferably, a flexible printed circuit board (FPC), a build-up multilayer wiring board and the like, using the cushioning pad.
2. Description of the Background Art
When a printed circuit board is formed by hot pressing, a method is used in which an object to be pressed is put between heating platens, and a prescribed pressure and heat are applied thereto. In order that uniform pressure and heat are applied to the entire surface of the object of pressing, such pressing is performed with a flat, plate-shaped cushioning pad interposed between the object of pressing and the heating platen.
Conventionally, silicon rubber or fluoro rubber has been used as the cushioning pad.
Silicon rubber, however, has a problem that low-molecular gas of siloxane contained in the rubber and bleeding are generated, possibly staining a printed circuit board or the like, which is the object of pressing. Such stain deteriorates printing characteristic onto the printed circuit board, or deteriorates adhesion of the printed circuit board with other member such as a reinforcing plate, for example. Such tendency is accelerated as the cushioning pad is used longer. Such a stain presents a significant defect especially when the object of pressing is a precision apparatus component (for example, a flexible printed circuit board for a hard disc drive, or a build-up multilayer wiring board). Further, silicon rubber also has insufficient mechanical strength.
Japanese Patent Publication No. 7-12617 (Japanese Patent Laying-Open No. 6-278153) discloses an example of the cushioning pad using fluoro rubber. Compositions of the cushioning pad disclosed in this publication are represented as Comparative Examples 1 and 2 in Table 1.
As fluoro rubber is used, the cushioning pad disclosed in the aforementioned publication has improved heat resistance. In addition, the cushioning pad has superior cushioning property, thermal conductivity, durability and so on.
When the cushioning material and an object of pressing (for example, an FPC) having unevenness on its surface are brought into contact with each other directly or with a film or the like interposed, however, there is a problem that the unevenness tends to remain on the surface of the cushioning pad. In other words, the cushioning pad has insufficient strain recovery.
When hot pressing involves fluidization-setup of an adhesive, especially when the cushioning pad described above is used for joining a base film and a surface film with an adhesive in manufacturing the flexible printed circuit board (FPC), there is a problem that voids tend to remain in the FPC as the object of pressing.
The present invention was made to solve the above described problems. An object of the present invention is to provide a hot press cushioning pad having superior strain recovery, superior conformability, not generating any void in the object pressing and not generating low-pad, molecular gas or bleeding, and to provide rubber applied for the cushioning pad, manufacturing method thereof, and a method of manufacturing a printed circuit board using the cushioning pad.
The inventors of the present invention studied strain recovery of the rubber for hot press cushioning pad considering relation between press temperature and press cycle of hot press, fluid behavior of the adhesive inside the object of pressing and so on, and found that the rubber exhibited superior strain recovery when tan xcex4 of the rubber for the cushioning pad under condition of use of hot press was 0.04 or lower.
According to an aspect of the present invention, the rubber has loss tangent value (tan xcex4) of 0.04 or lower measured by dynamic viscoelasticity measurement, when the temperature condition is set to the press temperature for hot press and frequency condition is set to the time corresponding to one press cycle of the hot press. The press temperature of hot press mentioned above refers to the highest temperature of the heating platen in one press cycle.
When the rubber is used as the cushioning pad for hot press and an object of pressing having unevenness on its surface (for example, an FPC) is hot-pressed, unevenness is not left on the surface of the cushioning pad. Therefore, superior cushioning property can be maintained for a long period of time.
The press temperature is 150xc2x0 C. to 300xc2x0 C., and the time corresponding to one press cycle is 1 to 2xc3x97104 seconds.
Preferable temperature condition is from 150xc2x0 C. to 250xc2x0 C., and preferable frequency condition is 1 to 1xc3x97103 seconds.
Under such conditions it is possible to adhere the base film and the surface film in manufacturing the flexible printed circuit board performed in a relatively short press cycle. Therefore, when the rubber having tan xcex4 value of at most 0.04 is used as the cushioning pad under the above described conditions, superior strain recovery of the cushioning material is exhibited in the step of adhering the surface film, in manufacturing the flexible printed circuit board.
The rubber in accordance with another aspect of the present invention is for a cushioning pad used when objects of pressing are laminated and integrated by fluidization-setup of an adhesive by hot pressing, and the rubber has loss tangent value (tan xcex4) of 0.04 or lower measured by dynamic viscoelasticity measurement when frequency condition is set to the time period from the start of fluidization to the end of setup of the adhesive.
When the rubber is used as the hot press cushioning pad and objects of pressing (for example, FPC) laminated and integrated by fluidization-setup of an adhesive are hot-pressed, uneven pressure generated when the adhesive is fluidized can quickly be made uniform. As a result, generation of voids can effectively be prevented.
The adhesive here refers to one for joining two layers through the change of state of fluidization-setup as well as one which forms a layer by itself after fluidization-setup. Such adhesive may be a thermoplastic material or a thermosetting material. More specifically, it may be a hot melt adhesive or a pre-preg.
Generally, the press temperature mentioned above is 150xc2x0 C. to 300xc2x0 C., and the time period from the start of fluidization of the adhesive until the end of setup of the same is from 1 to 2xc3x97103 seconds.
Preferably, durometer hardness of the rubber in accordance with JIS (Japanese Industrial Standards) K6253 is A75 or lower.
Accordingly, as can be seen from Table 2, the durometer hardness can be made smaller than in the prior art, and hence conformability can be improved. As a result, by the synergistic effect with the aforementioned superior strain recovery, generation of void between the surface film and the base film can effectively be suppressed when a flexible printed circuit board having unevenness or level difference on its surface is adhered with a surface film.
Preferably, the rubber in accordance with the present invention mainly consists of fluoro rubber.
Therefore, a cushioning pad having superior heat resistance, high mechanical strength and almost free of exudation of mixed materials is provided.
Preferably, the rubber is a fluoro rubber composition containing fluoro rubber component of polyol vulcanization system, a vulcanizing agent, a vulcanization accelerator and an acid acceptor. The sum of the fluoro rubber component, the vulcanizing agent and the vulcanization accelerator occupies at least 80 vol. % of the entire fluoro rubber composition.
As the fluoro rubber of polyol vulcanization system is used, compression set of the cushioning pad can be made smaller, elongating the life of the cushioning pad. When bisphenol AF is used as the polyol vulcanizing agent, heat resistance can more effectively be improved.
The rubber in accordance with another aspect of the present invention is a fluoro rubber composition containing fluoro rubber component of polyol vulcanization system, a vulcanizing agent, a vulcanization accelerator and an acid acceptor. The sum of the fluoro rubber component, the vulcanizing agent and the vulcanization accelerator occupies at least 80 vol. % of the entire fluoro rubber composition.
The inventors have found that by such composition, it becomes possible to provide rubber of which tan xcex4 value is 0.04 or lower. When such a rubber is used as the cushioning pad for hot press, a cushioning pad having superior strain recovery as described above is obtained.
The hot press cushioning pad in accordance with an aspect of the present invention includes rubber having tan xcex4 value of 0.04 or lower when the temperature condition is set to the press temperature of hot pressing and the frequency condition is set to the time period corresponding to one press cycle of hot press.
Therefore, even when used repeatedly for hot press, superior strain recovery is exhibited, and superior cushioning property can be maintained for a long period of time.
In accordance with another aspect, the hot press cushioning material of the present invention is used for laminating and integrating objects of pressing by fluidization-setup of an adhesive by the hot press, and includes rubber having loss tangent value (tan xcex4) of 0.04 or lower, measured by dynamic viscoelasticity measurement when the temperature condition is set to the press temperature of hot press and the frequency condition is set to the time period from the start of fluidization to the end of setup of the adhesive.
Therefore, when objects of pressing (for example, FPC) to be laminated and integrated by fluidization-setup of the adhesive are hot-pressed using the cushioning pad, uneven pressure generated when the adhesive is fluidized can quickly be made uniform. As a result, generation of voids can effectively be suppressed.
In accordance with a still further aspect, the hot press cushioning pad of the present invention is of a fluoro rubber composition containing fluoro rubber component of polyol vulcanization system, a vulcanizing agent, a vulcanization accelerator and an acid acceptor, with the sum of the fluoro rubber component, the vulcanizing agent and the vulcanization accelerator being at least 80 vol. % of the entire fluoro rubber composition.
As the hot press cushioning pad is of the rubber of the aforementioned composition, strain recovery is improved.
Preferably, the surface of the rubber is subjected to releasing treatment.
Further, one or more layers formed of the aforementioned rubber, and at least one layer selected from the group consisting of woven fabric, non-woven fabric, paper, film, foil, sheet and plate may be laminated and integrated. The woven fabric, non-woven fabric, paper, film, foil, sheet and the plate is preferably a single or composite material selected from synthetic resin, natural rubber, synthetic rubber, metal and ceramics.
Preferably, at least one surface layer of the cushioning pad is formed of the rubber. More preferably, the surface of the rubber is subjected to releasing treatment.
The method of manufacturing the rubber for hot press cushioning pad in accordance with the present invention includes the following steps. A low material rubber, a vulcanizing agent, a vulcanization accelerator, an acid acceptor and a plasticizer are kneaded to prepare kneaded compound. The kneaded compound is processed to a sheet to form an unvulcanized rubber sheet. The unvulcanized rubber sheet is subjected to primary cure to provide vulcanized rubber sheet. The vulcanized rubber sheet is subjected to secondary cure at a temperature not lower than the boiling point of the plasticizer, so as to volatilize the plasticizer.
By the secondary cure, vulcanization is completed, excessive vulcanizing agent is dissolved, gas generated during vulcanization is dissipated, and residual strain can be eliminated. In addition, strain recovery is improved. Further, as the plasticizer can be volatilized by the secondary cure, exudation of the plasticizer during the use of the cushioning pad can be prevented.
By the secondary cure, preferably, the value of tan xcex4 is set to 0.04 or lower.
The method of manufacturing a printed circuit board in accordance with an aspect of the present invention is for manufacturing a printed circuit board by placing a material of a printed circuit board and the cushioning pad stacked and placed between heating platens and heated pressed, in which the cushioning pad contains rubber having tan xcex4 value of 0.04 or lower, when the temperature condition is set to the press temperature of the hot press and the frequency condition is set to the time corresponding to one press cycle of the hot press.
The method of manufacturing a printed circuit board in accordance with another aspect of the present invention is for manufacturing a printed circuit board in which printed circuit board material having laminated structure and the cushioning pad are stacked and placed between heating platens and the printed circuit board material is laminated and integrated by fluidization-setup of an adhesive by hot-pressing, and the cushioning pad contains rubber having loss tangent value (tan xcex4) of 0.04 or lower, when measured by dynamic viscoelasticity measurement with the temperature condition set to the press temperature of the hot press and the frequency condition is set to the time period from the start of fluidization to the end of setup of the adhesive.
The method of manufacturing a printed circuit board in accordance with a still further aspect of the present invention uses a cushioning pad of the following composition. The cushioning pad has a fluoro rubber composition containing fluoro rubber component of polyol vulcanization system, a vulcanizing agent, a vulcanization accelerator and an acid acceptor, with the sum of the fluoro rubber component, the vulcanizing agent and the vulcanization accelerator being at least 80 vol. % of the entire fluoro rubber composition.
By the method of manufacturing the printed circuit board, it becomes possible to manufacture a printed circuit board of high quality free of void or shrink, having high smoothness and not susceptible to staining.
The printed circuit board has uneven pattern on its surface.
The printed circuit board is, for example, a flexible printed circuit board or a build-up multilayer wiring board.
The cushioning pad including the rubber in accordance with the present invention is effective for hot pressing such a printed circuit board.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.